The oxygen-selective self-referencing probe is being used to test if differences in oxygen availa-bility within a microscopic preparation are responsible for observed inhibition in embryo develop-ment. The experiments are using a classic model of developmental organization, the echninoderm ooyte and embryo and follow observations by Inoue that embryo position, in a confluent mass, causes asynchronous growth. The initial research has been oriented toward documenting the changes in oxygen flux into unfertilized eggs and embryos, and seeing how oxygen consumption changes as the embryo develops. To date we have been able to show that a significant increase in the oxygen influx is associated with the fertilization process. Furthermore, we have been able to measure the kinetics of these changes in real time. While experiments are in the early stages, this system has shown much promise for further research. The echinoderm system is extremely easy to work with. Millions of gametes can be obtained from a single animal and these gametes remain viable for hours in seawater at room temperature. Therefore, oocyte fertilization and zygote formation can be initiated at the convenience of the investigator and the exact time can be noted. The studies documenting changes in oxygen consumption in mouse embryos is complemented by this research. It will undoubtedly be valuable to compare the findings from these. The starfish embryo develops in reasonably well oxygenated waters whereas the mouse embryo develops in the hypoxic uterine environment. Overall these studies will contribute to our understanding of the metabolic changes associated with early embryonic development.